1. Field of the Art
This invention relates to a polarizing filter which is capable of transmitting polarization of a specific direction alone, and a method for fabricating such a filter.
2. Prior Art
Polarizing filters are used in optical devices such as optical pickups and liquid crystal projectors or in image pickups of digital cameras, for the purpose of selectively transmitting either p- or s-polarized component in mixed polarization light. As for example of polarizing filter, Japanese Laid-Open Patent Application H5-19208 discloses a polarizing filter which is formed by bonded in an array a plural number of polarized beam splitters each with a polarization separating filter layer to transmit or reflect polarized components of input light according to the direction of polarization. In just-mentioned Japanese Laid-Open Patent Application H5-19208, in order to impart functions as a polarizing filter (which is called “a polarized beam splitter array” in No. H-19208), a plural number of polarized beam splitters, each formed with a polarization separation coating at an inclination angle of 45 degrees, are arrayed and bonded together in such a way that polarization separating filter layers on two adjacent polarized beam splitters are disposed in perpendicularly intersecting relation with each other.
In the case of the polarizing filter in above-mentioned Japanese Laid-Open Patent Application H5-19208 having a plural number of polarized beam splitters bonded together to make up a polarizing filter, the polarization separating filter layers on adjacent polarized beam splitters can be brought into linear contact only when they are bonded with perfect accuracy, and continuity of polarization separating filter layers can be damaged even by a slight deviation of joining surfaces. If a gap exists in a polarization separating filter layer of a polarizing filter, leakage of incident light takes place at the gap, where part of incident light is simply leaked through without undergoing the separating action of the polarizing filter. That is to say, leaked light still contains p- and s-polarized components in a mixed state, because the polarizing filter is unable to perform its functions to a sufficient degree due to a flaw in the filter layer.
Further, at the time of producing a polarizing filter by bonding together a number of polarizing beam splitters, a step-out portion is formed at a joint portion of two adjacent polarized beam splitters if one polarized beam splitter is deviated relative to the other one in a direction parallel with direction of propagation of incident light. If such a step-out portion is formed, an edge portion (an edge line) of one polarized beam splitter stands out distinctively in such a degree as to invite cracking or chipping damages, which might lead to a loss of part of a polarization separating filter layer on the stepped-out edge portion. In that case, a gap is opened up in the lost portion, and, as mentioned above, unseparated light, i.e., a mixture of p- and s-polarizations, is transmitted directly through the gap. That is to say, the polarizing filter no longer can perform its function satisfactorily.
If such a flawed polarizing filter is applied to a liquid crystal projector, for example, a mixture of p- and s-polarizations is transmitted through the lost part of a polarization separating filter layer to darken or dim out a brightness level of picture images projected on a screen. In a case where a polarizing plate is located at a position posterior to the polarizing filter, a mixture of p- and s-polarizations is absorbed by the polarizing plate. Since a service life of a polarizing plate is dependent on an accumulated light volume, leaks of mixed light causes increases to the light volume absorbed by the polarizing plate which is located posterior to the defective polarizing filter, shortening the service life of the polarizing plate.
On the other hand, in the case of the polarizing filter which is composed of an array of polarized beam splitters, if a stepped-out portion is formed at a joint of adjacent polarized beam splitters as mentioned above, the polarizing filter as a whole is distorted into either concave or convex shape without retaining a planar shape. Such a distorted polarizing filter comes to have lens effects, instead of purely performing its intended function alone, i.e., the function of separating p- and s-polarizations.
Thus, when fabricating a polarizing filter of this sort, perfection in accuracy is required at the time of bonding a number of polarized beam splitters in an array, which however is extremely difficult because polarized beam splitters are very small in size. Especially, as described in Japanese Laid-Open Patent Application H5-19208, for compactification of optical systems as a whole, there are strong demands for polarizing filters which are reduced in size, weight and thickness, despite the problem that it is extremely difficult to bond together small polarized beam splitters, in addition to a problem that a production cost becomes higher when higher accuracy is required in a fabrication process.